Saturn’s moon Mimas may be the newest, if not the most unlikely member of the solar system’s family of icy bodies with internal liquid water oceans, according to a new study by Alyssa Rose Rhoden of the Southwest Research Institute and Matthew E. Walker of the Planetary Science Institute. These bodies, which include Europa, Enceladus, and possibly Pluto and Triton, are all considered prime targets for exploration due to their ability to maintain potentially life-sustaining water under their outer ice shells at a wide range of distances from the Sun.

What makes Mimas unusual is what it lacks in comparison with all other known ocean-bearing moons: active geology. Mimas is small (less than 400 kilometers in diameter) and heavily cratered, suggesting that it has remained frozen and geologically inactive for many millions of years despite the powerful tidal forces it experiences as it orbits Saturn. Observations made by the Cassini spacecraft of Mimas’ orbital libration, or variation in its tilt over time, suggested that either its core was irregularly shaped or that an internal ocean could exist. For Mimas to maintain an internal ocean, but have an ice shell thick enough to remain frozen, would require a careful balance of tidal heating, as its small size means it would quickly lose any heat that is produced.  

Rhodes and Walker used tidal heating models to find the balance between these two constraints and found that an ice shell between 24 and 31 kilometers thick produced a good fit for the data. Because the thickness of the ice shell is related to the surface heat flux, a future mission with an instrument similar to Juno’s microwave radiometer could verify this finding. The confirmation of an ocean on such an unlikely world would greatly expand our understanding of how prevalent these important ocean moons may be in the solar system and beyond. READ MORE